This invention relates generally to the field of bicycles, and more particularly to derailleur assemblies for bicycles.
A derailleur is a mechanism for shifting gears on a bicycle that operates by moving the bike chain from one set of exposed gears to another. Conventionally, derailleurs have been secured to bicycle frames or other immobile parts to prevent the derailleurs from moving with respect to the bike chain. This is important because the angle formed between the bicycle seat tube and the chain stay (the "chainstay angle") must stay relatively constant for the derailleur to operate as designed. A brochure from Shimano, one of the leading manufacturer of derailleur assemblies, states that the chainstay angle must be fixed in a range of three degrees, i.e., from 63.degree. to 66.degree., for the specified derailleur to be operative. Therefore, as can be deduced, the position of a derailleur with respect to a bicycle chain must be kept within a tolerance of a few degrees for the derailleur to operate properly.
With the advent of rear suspension forks on bicycles, the chain is not always maintained in the same position with respect to the frame. For example, when the rear wheel moves upward, the angle between the seat tube (from which the derailleur is generally suspended) and the chain decreases. In this position, the derailleur is not optimally positioned to change gears. Hence, a need for a derailleur mounting assembly that allows the derailleur to remain at a constant angle relative to the bike chain, even though the rear fork allows the rear bicycle wheel to move, has arisen.
The present invention overcomes the above-described disadvantages of the prior art by providing a mounting for a derailleur assembly that is pivoted about the bicycle crank axis and that moves relative to the movement of the rear wheel and the chain. The pivotability of the derailleur assembly about the crank axis insures that the derailleur's position remains constant with respect to the bike chain during operation of the bicycle. Thus, this design allows a bicycle to incorporate a rear suspension fork without any problems resulting in gear-shifting capabilities.